LEDs (light emitting devices) are used in a wide variety of electronic devices as a display for example, to convey information about the status of a device or as a monitor. The various types of LEDs include organic LEDs, semiconductor LEDs and liquid crystal displays. FIG. 1 shows a conventional arrangement 100 for a plurality of LEDs. The LEDs are arranged as a matrix with the plurality of LEDs connected in rows 11 and columns 13. An LED 101 of the matrix comprises a first terminal 102 and a second terminal 103 (e.g. an anode and a cathode). The first terminal 102 of each LED 101 in a column 13 are connected together and coupled to a column control input 150 while the second terminal 103 of each LED in a row 11 are connected together and coupled to a row control input 180. In order to enable a specific LED, the row and column corresponding to that LED are activated through the respective row and column control input. As illustrated in FIG. 1, each column of LEDs 13 is connected to a column select transistor 110. The column select transistors 110 selectively couple or decouple the columns to a reference voltage 130 (e.g. Vcc which represents a logic 1) depending on the states (active or inactive) of the column control inputs 150 coupled to transistor gates 105. For the arrangement in FIG. 1, providing a logic 0 (active signal) at a column control input 150 switches on the pnp column select transistor 110 and the first terminals 102 of the LEDs in that column are coupled to Vcc 130 (logic 1). Similarly, a row 11 is also activated by providing a logic 0 (active signal) at the row control input 180. The voltage established across the LED with its row 11 and column 13 activated switches it on. The number of voltage levels in an active signal used to light up an LED depends on the type of display. For example, a monochromatic display only requires a one bit digital signal as the LED is either on or off. In a grayscale or color display, a multilevel digital or an analog signal is used to achieve a range of a specific color between full on and full off.
The conventional arrangement as described above requires a separate control input for each column and row in the matrix. Therefore the number of LEDs that can be driven is limited by the formula m*n, where m=number of column control inputs, n=number of row control inputs and the total number of control input lines required=m+n. This arrangement can prove to be inefficient when the LED matrix has a large number of rows and columns. For example, in order to implement a matrix comprising of 400 LEDs, 40 control inputs or external connections are required. Correspondingly, if a chip is used to control the LED matrix, it will be required to have 40 control pins just to address the matrix. As evident from above, the conventional arrangement is not conducive to miniaturization of chip size and presents a problem particularly in portable electronic devices such as mobile phones, radios and chipcards.
Some attempts to increase the number of LEDs controlled by a given number of pins have led to the use of external decoders and decoding switches (as taught in European Patent Nos. EP597226-A1 and EP809228-A2 which are herein incorporated by reference in their entirety into the present disclosure). Generally, in these implementations, a plurality of decoding switches are used wherein each decoding switch is coupled to a row/column control pin and a group of individual rows/columns. An external decoder is connected to the plurality of decoding switches for selecting an addressed row/column within a group. This approach, however, has the disadvantage of increasing the overall Bill of Materials (BOM) as there is a need for an external decoder and decoding switches for each group of rows/columns.
It would be desirable to provide an arrangement that would increase the number of LEDs that can be controlled by a given number of inputs while not increasing the BOM significantly.